This invention relates to two-way communications systems and, more particularly, to polled data networks employing high speed modems interconnected by telephone lines.
Communications networks for interconnecting digital devices such as computers and facsimile machines traditionally employ a modem (modulator/demodulator) at each communications site. Each modem includes transmitter and receiver circuits which convert digital data to analog signals suitable for transmission and reception over telephone lines. In order to increase the efficiency of such systems, there is a trend toward modem designs which are able to transmit and receive data at ever higher data rates.
Traditional modem designs provide data communications rates of 300 to 1200 bits per second (bps). At data rates of 2400 bps and above, the signal distortion characteristics of the telephone lines becomes a major design consideration. Such distortion characteristics include amplitude and phase distortion which must be compensated for, or equalized, in order to effect high speed communications with low data errors.
Traditionally, line distortion is compensated for by employing adaptive filters in the receiver portions of each modem in the system. These filters and their associated circuitry, sometimes referred to as autoequalizers, generally include tapped delay lines, where each tap is associated with a variable gain coefficient which is adaptively set in response to a received data sequence. The adaptive circuits may use one of a number of convergence techniques to iteratively vary the filter gain coefficients until the filter output signal is compensated for the distortion characteristics of the line.
One of the drawbacks of the prior art auto equalizer methods is the time required (referred to as equalizer time) for the equalizer circuits to converge to the desired signal compensation. For example, a typical 4800 bps modem may require 50 milliseconds or more of equalizer time, and a 9600 bps modem employing state of the art equalizer techniques may still require 20 milliseconds or more of equalizer convergence time. Since additional data cannot be received until the equalizer has converged, this time interval represents the waste of valuable data transmission time, as well as the additional expense of telephone line charges.
The above described problem is compounded in polled data networks requiring a large number of relatively short transmissions to be received from many remote sites. In such instances, it may well be that the equalizer convergence time, which is required for each new transmission received at a remote site, is substantially longer than the actual data transmission time.
Accordingly, it is an object of the present invention to provide a new and improved communications system.
It is another object of the present invention to provide a polled data network employing a master and remote modems, in which the communications line equalizer cnvergence time is eliminated for signals received by the master modem in the system.